Heat the oven to 450°F/230°C. Heat 2 tablespoons of the olive oil in a sauté pan and cook the onions until soft, about 5 minutes. Add the garlic, bay leaf, and pepperoncino, and cook one minute. Then add the tomatoes and cook to a thick, chunky sauce, about 30 minutes. Remove the bay leaf, season with salt and pepper, and stir in the fresh herbs.
While the sauce cooks, brush the eggplant slices on both sides with olive oil and lay in a single layer on a baking sheet (you’ll need to work in batches). Bake until golden and soft, about 15 minutes; turn and grill the other side, about 10 minutes. Remove and set aside. Turn the oven down to 375°F/190°C. Mix together the bread crumbs with the chile powder.
In a large gratin dish, starting with just the thinnest smear of tomato sauce over the bottom, make layers of eggplant, sauce, then breadcrumbs. Repeat until the dish is full. Scatter the cheese over top. Bake until the gratin is heated through, and the top golden, about 30 minutes. Serve.